deck_11268041 Flashcards
what is a MAGNETIC FIELD?
where a magnet/magnetic material experiences force
examples of MAGNETIC MATERIAL
iron, steel, nickel, cobalt
what happens to magnetic materials when they’re next to magnets? why?
ATTRACTED because of INDUCED MAGNETISM (domains line up)
how do you DESTROY A MAGNET (remove magnetism)?
make the domains point in different directions:- hammer it- heat it- wrap a coil and pass alternating current
why doesn’t a normal piece of magnetic material (e.g. iron) act like a magnet?
domains point at different direction and cancel out
how do you MAKE A MAGENT from magnetic material?
make the domains line up:- stroking- put next to another magnet- wrap coil + pass current
difference between IRON and STEEL’S MAGNETISM
IRON: soft - easy to magnetize and demagnetize (immediately loses magnetism when magnetic field is removed)STEEL: hard - hard to magnetize and demagnetize (retains magnetism even after magnetic field is removed)
property of INDUCED MAGNETS
TEMPORARY MAGNETISM, loses magnetism when magnetic field is removed
how do you SHOW THE DIRECTION of MF?
small compasses in a circle → needles point in the direction of field lines
magnetic field lines always go from … to … outside the magnet
N to S
DENSER magnetic field lines means…
STRONGER magnetic field
uses of HARD magnetic materials
making permanent magnets, loudspeaker
uses of SOFT magnetic materials
motors, generators, electric bells, transformer cores
differences between ELECTROMAGNETS and PERMANENT MAGNET
PERMANENT MAGNET:- constant strength of magnetic field- can’t be switched on/off- N and S can’t be changedELECTROMAGNET:- variable strength of magnetic field- can be quickly switched on/off- N and S can be changed by changing direction of current
how do you measure the strength of an electromagnet?
measure the mass of paper clips it can hold
do you use iron or steel as the core in the LOUDSPEAKER? why?
STEEL cuz its HARD magnetic material - doesn’t lose magnetism easily
do you use iron or steel as the core in the RELAY? why?
IRON cuz its SOFT magnetic material - can change magnetism easily
what happens when electric current flows through a wire?
MAGNETIC FIELD is created
difference between A.C. and D.C.
D.C. → doesn’t change direction → constant MF| A.C. → changes direction → alternating MF
how does the pattern of magnetic field look like outside a SOLENOID?
same as bar magnets
how does the pattern of magnetic field look like outside a CURRENT-CARRYING WIRE?
circular, perpendicular to wire
how do you make an electromagnet stronger?
• increase current/voltage• increase number of turns in coil→ increased strength of MF
how do you determine the DIRECTION OF MF in a CURRENT-CARRYING WIRE?
right hand rule:• thumb - direction of current• fingers - direction of MF
how do you determine the DIRECTION OF MF in a SOLENOID?
right hand rule:• thumb - points to N pole• fingers - direction of MF
explain the MOTOR EFFECT
power supply gives VOLTAGE across coil → CURRENT flows through coil → MF induced around coil → INTERACTS with permanent magnet’s MF → interaction creates FORCE → coil spins
MOTOR EFFECT:| how to increase the size of force?
- increase current/voltage* increase turns of coil* stronger magnet → stronger MF
MOTOR EFFECT:| how to reverse the direction of force?
- reverse direction of current (battery)| * reverse direction of MF
uses of motor effect
hairdryer, blender, electric drill, etc.
MOTOR EFFECT:| how to determine direction of force/current/MF if others are known?
right-hand slap rule:拇指 - Current 其他手指 - Magnetic Field手心 - Force fleming’s left-hand rule:拇指 - F (force) 食指 - B (magnetic field) 中指 - I (current)
energy transfer in D.C. MOTOR
electrical → kinetic
D.C. MOTOR:| what component is needed? why?
SPLIT-RING COMMUTATOR| ensures current always flow the same way → continuous rotation of coil
explain the GENERATOR EFFECT (also known as electromagnetic induction)
electric conductor experiences CHANGE IN MF → VOLTAGE is induced → CURRENT is generated
GENERATOR EFFECT:| explain 2 ways in which you can induce current + observation to show current is induced
move a wire (connected to galvanometer) up and down between N and S poles of a magnet → cuts through MF lines → induces V+I → galvanometer needle movesmove a magnet in and out of a coil (connected to galvanometer) → cuts through MF lines → induces V+I → galvanometer needle moves
GENERATOR EFFECT:| what device do you use to show current is induced?
galvanometer
GENERATOR EFFECT:| how to increase the size of induced current?
- faster motion* more turns of coil* stronger magnet → stronger MF
A.C. GENERATOR:| why is an alternating current (a.c.) produced instead of direct current (d.c.) ?
one side of the coil moves up → induces VOLTAGE in one direction → induces CURRENT in one direction… rotation continues … same side of the coil moves down → induced VOLTAGE reverses direction → induced CURRENT reverses direction*this repeats and produces a constantly changing current
A.C. GENERATOR:| when is the induced V+I at maximum?
when coil moves PERPENDICULAR TO MF → cutting MF
A.C. GENERATOR:| when is there no induced V+I at all?
when coil moves PARALLEL TO MF → not cutting MF
A.C. GENERATOR:| what components are needed? why?
SLIP RING + BRUSH| maintain contact between coil and external circuit
energy transfer in A.C. GENERATOR
kinetic → electrical
uses of generator effect
wind turbines
what is the function of a TRANSFORMER?
change V to a bigger/smaller value
STEP-UP transformer:| function, look, advantage?
increases voltagemore coils on secondary sideminimizes energy loss
STEP-DOWN transformer:| function, look, advantage?
decreases voltageless coils on secondary sidesafe to use
why does a STEP-UP transformer MINIMIZE ENERGY LOSS?
(constant power, P=IV)| V increase → I decrease → lower I wastes less energy
explain how a TRANSFORMER works
Alternating Vp → Alternating Ip → Alternating MF → Alternating MF induced in iron core → Vs is induced → Is flows
TRANSFORMER:| what material is the core made of? why?
IRONcuz it is SOFT MAGNETIC MATERIAL → can be magnetized (MF can be induced)→ easy to magnetize and demagnetize
TRANSFORMER:| what’s the equation relating V and N (number of coil) on primary and secondary sides?
Vp/Np = Vs/Ns
TRANSFORMER:| what’s the equation relating V and I on primary and secondary sides?
P = Ip• Vp = Is • Vs
TRANSFORMER:| what type of current is needed at primary side? why?
A.C.| cuz it needs to induce a CHANGING MF
